The modern brake master cylinder practice of placing primary and secondary pistons in tandem in a single master cylinder tends to increase the length and weight of the brake master cylinder. Increasing size and weight of accessories runs counter to the desire for smaller and lighter vehicles. In addition, in large vehicles such as trucks, the mass becomes too great to support in the usual cantilever fashion. The additional support bracket required further increases vehicle weight.
In a conventional brake master cylinder, the piston must be longer than its stroke and have a resilient seal near each of its ends. A passage for compensation brake fluid from a fluid reservoir communicates with the perimeter of the piston between the two seals. The two seals must continue to enclose the fluid passage throughout the limits of motion of the piston. Thus the cylinder must necessarily be longer than twice the axial piston motion.
The problem is multiplied in a tandem brake master cylinder containing two pistons. Normally, failure in one of the two quasi-independent brake systems allows the associated piston to pass to its limit before the remaining piston may begin supplying brake pressure. Thus the piston located deeper in the bore must be longer than its axial motion. The piston located nearer the mouth must be long enough, not only to cover its own normal axial motion but also must continue to work as it follows the deeper piston into the cylinder in the event of failure in the part of the system associated with the deeper piston. Thus the shallower piston must have a length exceeding the sum of the axial motions of the deeper and shallower pistons. The total length of the cylinder can thus exceed five times the axial displacement of one piston.
Prior solutions have concentrated on removing the compensation function from the cylinder bore and transferring it to compensation valves outside the bore. For example, U.S. Pat. Nos. 3,818,706 and 3,879,947 disclose hinged unidirectional valves which allow the entry of brake fluid into the cylinder during the partial vacuum created in the cylinder during brake release and which are mechanically tilted into the open position by leverage exerted by the pistons as they return to their fully brakes-off position. When tilted open in this manner, excessive brake fluid in the cylinder is relieved to the reservoir during brakes off. Although this solution succeeds in shortening and consequently lightening the brake master cylinder, the hinged tilt valves are prone to allowing dirt particles to wedge into their seat regions. Lacking large fluid flow or motion to dislodge the dirt, the tilt valve can fail to seal. This failure allows fluid pressure bleedoff into the brake fluid reservoir rather forcing the pressure into the brakes.
The prior art fails to disclose a brake master cylinder in which the sealing and compensation functions are performed by limited stroke auxiliary pistons.